1. Field of the Invention
The present invention relates to new organic materials used in organic transistors. More particularly, the present invention pertains to novel organic compounds facilitating the ohmic contact between semi-conducting layer and electrodes and use in organic thin film transistors.
2. Description of the Related Art
Thin film field-effect transistors (FETs) comprise the basic building blocks for microelectronics. A FET has three electrodes (e.g., source, drain, and gate electrodes), an insulator layer, and a semiconductor layer. A FET operates as a capacitor where a semiconductor layer is a conducting channel between two electrodes, i.e., the source and the drain. The density of charge carriers in the channel is modulated by voltage applied to the gate electrode, so that the electric charge flow between the source and the drain electrodes can be controlled by voltage applied to the gate electrode.
There has been great interest recently in the development of FETs using organic semi-conducting materials. With organic semi-conducting materials in FETs, electronic devices can be manufactured in a printing method, such as screen-printing, ink-jet printing, and/or micro-contact printing. In addition, these materials can be processed at a much lower substrate temperature and with little or no vacuum involved, as compared to the typical inorganic semi-conducting materials. Therefore, electronic devices, including FETs, that use organic semi-conducting materials can be flexible and less costly to produce compared with using inorganic semi-conducting materials.
Different types of organic materials such as small molecule, polymers and oligomers have been tested as organic semi-conducting materials in FETs since the 1980s. With concerted effort in this area, the performance of an organic FET has improved from 10−5 cm2/V s to 1 cm2/V s in terms of charge carrier mobility in a FET (J. M. Shaw, P. F. Seidler, IBM J. Res. & Dev., Vol. 45, 3 (2001)). The performance of an organic transistor is now comparable to that of an amorphous silicon transistor, so that organic transistors can be applied to E-paper, smart cards, and possibly displays.
Important electronic devices, which can be manufactured with semi-conducting organic materials, include organic light emitting diodes, organic solar cells, and organic transistors. In these devices, the electrical contact between the semi-conducting organic materials and electrodes is crucial to improving the performance of these devices. For example, the charge-carrier injection layers, such as hole-injection and electron-injection layers, are interposed between semi-conducting layers and electrodes to improve the performance of organic light emitting diodes. Even though the operation mode of the organic transistor is different from that of the organic light emitting diode, electrical contact between the semi-conducting layer and source and drain electrodes has a profound effect upon the performance of the organic transistor. It has been reported that the performance of the organic transistor depends upon the materials of choice for the source/drain contact metal (Y. Y. Lin et al. Materials Research Society Symposium Proceedings (1996), 413(Electrical, Optical, and Magnetic Properties of Organic Solid State Materials III), 413-418. CODEN: MRSPDH ISSN: 0272-9172). In this report, the metals with high work function (Pd, Pt, and Au) showed the best performance while the metal (Al) with relatively low work function showed a significantly degraded performance. Therefore, metals with high work function such as gold have been used for the source/drain electrode materials in most organic transistors. However, high work metals, which are novel metals, are expensive and hard to process using industrial methods, thus restricting their application and structure in organic transistors.